Q&A: Dehumidifying the Air

Flashing a Stucco Chimney

Q.Can a dehumidifier be added
to a forced-air system? If so, what should a contractor
know about specifying this?

A.It is possible to add a
dehumidifier to a forced-air system, but there is
really no good reason to do so. Moisture vapor
moves freely throughout the house, so humidity
control in one place will work for the entire
house. This means that a portable dehumidifier is
usually enough to control humidity when you
can’t do so by eliminating the moisture
source or by using an exhaust fan. (These should
always be your first choices for controlling
moisture.) A dehumidifier with a 40-pint capacity
can handle most houses.

A dehumidifier is essentially an air conditioner
that discharges the warm air back into the house
rather than exhausting it outside. Therefore, an
air conditioner can also be used to dehumidify a
house. But since an air conditioner only
dehumidifies when it is running, most systems
usually cool the house before the humidity is
brought under control. If little cooling is
desired, but you want to dehumidify the house,
it’s possible to size a central air
conditioner to a quarter or half the size needed
for normal cooling and run the system continuously
without overcooling the house.

In superinsulated houses with very low cooling
loads, we often recommend installing a small (5,000
Btu) window air conditioner somewhere in the house
to control humidity, and a central cooling system
to control temperature. The window unit can be
removed during the winter when dehumidification is
not needed.